The biaxially drawn, blow-molded bottle of a polyethylene terephthalate resin (hereinafter referred to as the PET resin) can be given a thin and uniform wall thickness because of distinguished characteristics of PET. Since such bottles are economical, have high resistance to contents and a high mechanical strength, and have good outer appearance, the bottles are widely used as liquid containers in various fields.
As described above, the PET bottle has a high mechanical strength despite its thin wall. However, since the body, a major part of the bottle, has a thin wall, the bottle is inconvenient in that a part of the body may falsely become dented and deform under a reduced pressure created inside the bottle and may give a marked damage to the outer appearance of the bottle. As a commercial product, the bottle may be quite poor in appearance.
Especially in recent years, widely spreading applications require the bottles to be hot-filled with beverages at a temperature in the range of 85 to 95° C. After the hot filling, the bottles are found to be at a greatly reduced inner pressure once the bottles have been cooled. Thus, there is an ever-increasing request for the bottles that can be prevented from being deformed under such a reduced pressure.
In the applications requiring sterilization of retort-packed foods, e.g., by heating the foods at 121° C. for 30 minutes after the bottle has been filled with the contents, the resin for molding the bottle must be resistant to this temperature, and in addition, the bottle should be able to stand up to severe depressurization.
In order for the PET bottle to be protected from the disadvantage of deformation under reduced pressure, various proposals have been made for the PET bottles. For instance, utility model laid open No. 1982-199511 discloses a number of deformable, slightly hollowed panel walls, which are disposed in the body of the bottle and easily become further dented inward so as to absorb a negative pressure created inside the bottle. Since the deformable panels become dented to a certain shape, other portions of the body are protected from false dented deformation under reduced pressure. Thus, the body of the bottle is prevented from showing poor outer appearance.
However, the deformable panel walls in the above-described conventional art has a problem in that the extent to which negative pressure can be absorbed is not sufficient, considering the extent of dented deformation created under the reduced pressure. This is because the deformable panels have been molded beforehand simply in the shape slightly dented inward so that the dented deformation may occur easily under the reduced pressure created inside the bottle.
Another problem of the deformable panel walls is that the body has a decreased buckling strength due to the existence of these deformable panels, which are molded by denting and deforming a part of the walls and which are equally spaced in a row around the circumference of the body.
Still another problem of the deformable panels is that the bottle sometimes looks poor in appearance. Since the deformable panel walls that become dented are longer than are wide, the portion of the body surrounded by the deformable panels looks quite lean as compared with other portions of the body, depending on the angle from which the bottle is viewed.
Lastly, there is a problem that the bottle becomes permanently deformed. All of those bottles causing a reduced pressure to be created inside are filled with hot liquid contents. Initially when the bottle is filled with the hot contents and sealed, the inside of the bottle is put under a pressurized condition. Therefore, the deformable panel walls are also required to have an ability to absorb a pressure, in addition to the ability to absorb a reduced pressure. Since these deformable panel walls have a shape of simply curved and dented panels, the panels cannot fully absorb the pressure. If a large pressure is applied, the deformable panels are not elastically inflated but are reversibly projected, and remain permanently deformed.
In spite of these many difficulties, fact is that the above-described deformable panels have been and are used in the bottles in most cases where an especially severe reduced pressure is derived from the hot filling using a temperature in the range of 85 to 95° C.
This invention has been made to solve the above-described problems observed in the conventional art. Thus, the technical problem of this invention is to eliminate the need to use the deformable panel walls and to find the body of such a shape that no false deformation, such as dented deformation, takes place in a portion of the body due to the hot filling or the reduced pressure created after the treatment of retort-packed foods. The object of this invention is to obtain a bottle that can inhibit the deformation caused by reduced pressure, has a high buckling strength, and is good in outer appearance.